This invention relates to low-voltage space thermostats which control operation of single-stage heating and cooling systems.
Typically, in a single-stage heating and cooling system, the heating system includes a low-voltage operated gas valve which controls the flow of gas to the furnace; the cooling system includes a contactor having a low-voltage coil and high-voltage contacts which contacts control energizing of the compressor; and the circulation system includes a fan relay having a low-voltage coil and high-voltage contacts which contacts control energizing of the fan which circulates the conditioned air.
The electrical power for energizing such low-voltage operated devices is provided either by a single transformer or by two separate transformers. If the heating and cooling system is installed as a complete unit, generally a single transformer is provided. Such a single transformer has the required volt-ampere output to operate all the low-voltage operated devices. If the cooling system is added to an existing heating system, sometimes an additional transformer is used.
Specifically, in a system for heating only, a fan relay is generally not provided since the fan is generally controlled directly by a thermal switch on the furnace. Therefore, in a system for heating only, the only electrical load on the transformer is the gas valve. When the cooling system is added, the electrical load on the transformer increases due to the addition of the fan relay and the contactor. If the existing transformer does not have the required volt-ampere output to operate all the low-voltage operated devices, an additional transformer is required. It is noted that generally, in anticipation of the existing transformer not having the required volt-ampere output to handle such additional electrical load, a transformer is provided as part of the hardware in an add-on cooling system package. If the transformer is required due to the increased electrical load, it is used; however, even if the additional transformer is not required, it is generally used so as to simplify the electrical wiring involved in the installation of the add-on cooling system.
It is desirable that a low-voltage space thermostat for controlling a single-stage heating and cooling system be constructed so as to enable it to be readily usable with either the single-transformer or two-transformer power source. While use with the single-transformer power source poses no problem, a potential problem can arise when used with the two-transformer power source. Such a potential problem is that the two transformers might be connected to the thermostat in such a manner so that they are out of phase with each other whereby the voltages at the secondary windings would aid each other and thereby effect an unacceptably high value of voltage potential between various wiring terminals in the thermostat. For typical transformers having a rated 24 volt RMS secondary voltage, this unacceptably high value is approximately 68 volts peak voltage.
The most commonly used prior-art approach to negating this potential problem has been to incorporate means for electrically isolating the secondary windings of the two transformers from each other. For example, in a known prior-art construction, typified in U.S. Pat. No. 4,049,973, five wiring terminals are provided in the thermostat. Two of the thermostat terminals, isolated from each other with respect to the internal circuitry of the thermostat by a multi-position system selector switch, are normally connected together at the terminals by a removable wire jumper. When the heating and cooling system uses a single transformer, the wire jumper is retained, and one end of the secondary winding of the single transformer is connected to one of the two jumper-connected terminals. The other end of the secondary winding is connected through the fan relay, gas valve, and contactor to the remaining three terminals. When the heating and cooling system uses two transformers, the wire jumper is removed, and one end of the secondary winding of one of the transformers is connected to one of the two terminals previously connected by the wire jumper, and one end of the secondary winding of the other transformer is connected to the other of the two terminals previously connected by the wire jumper. The other end of the secondary winding of one of the transformers is connected through the gas valve to one of the three remaining terminals, and the other end of the secondary winding of the other transformer is connected through the fan relay and contactor to the remaining two terminals. Since the two terminals previously connected by the wire jumper are electrically isolated from each other, the secondary windings of the two transformers are therefore also isolated from each other.
While the above-described construction negates the potential problem created by two transformers being connected out of phase, it uses five thermostat terminals. As will be described hereinafter, a particular thermostat, constructed so as to provide automatic changeover between heating and cooling, results in a construction which, for economy, uses only four terminals. The use of such a four-terminal thermostat with a single-transformer power source poses no problem. However, when such a four-terminal thermostat is used with a two-transformer power source, one end of the secondary winding of each transformer is connected to a single terminal and the other ends of the secondary windings are connected through the fan relay, gas valve, and contactor to the remaining three terminals, whereby, if the two transformers are out of phase, an unacceptably high voltage potential exists between certain ones of the three remaining terminals.
Correcting this out-of-phase condition requires simply that the voltage polarity of one of the transformers be reversed. This can readily be accomplished either by reversing the primary or secondary leads of one of the transformers at the transformer location or by reversing specific connecting wires at the thermostat. While there is no particular problem in correcting the out-of-phase condition, in the manner described above, there is a problem in detecting the existence of such out-of-phase condition. It is noted that the out-of-phase condition could be detected by checking the voltage, such as with a volt meter, between the various thermostat terminals. However, since thermostats are often installed by do-it-yourself homeowners who may not have equipment such as a volt meter, it is believed desirable that other detecting means, requiring no specific equipment, be provided.